Going from a solo game project to a team project

I have been working on a game partial time for 2 years now, I've done the code and assets myself, but eventually I understood that I would not be able to have nice looking assets without spending a lot of time into it. And that greeting a new team member with graphic designer skills on this project would be a good idea.

My project is purely casual ATM. While I would love to earn a bit of money from it, my main goal is to have something fun to play with my friends (it's a couch console game) and I'm not sure what I want for the release of the game.

Now that I'm about to greet a graphic designer in the game project I'm excited but I want to do things right.

What should I define prior welcoming a second member in the team to make things work? (Maybe the goal of the game in term of features? If we want to sell it or release it for free? If we sell it how we want to split the earnings?)

What if things goes wrong and we split, when I have worked 900h on the game and him 20?

Any other tips from your experience ?

(I realise that this open question might not be suited for this stack exchange, tell me if this is the case)